Hi Alan, assuming its a mikuni VM carburettor, im thinking the pilot jet bypass hole is blocked (If it has one) its a small hole in the bottom of the venturi, allows fuel through the pilot jet to exit behind the throttle slide when shut, also worth a look at the air jet. A rough guide to this working or not is to adjust the mixture screw, it should have a noticeable affect on engine speed at tickover, if blocked you will see no affect.